A quick trip from Seoul in South Korea towards the south east is the wonderful Namhansanseong National Park. A historical old city on top of mountain with dense forest, surrounded by fortified walls, used as the emergency backup castle for the Korean kings. It offers vast panoramic views of Seoul city, where you can get a sense of the scale of the city, with ~10 million inhabitants. There are various attractions, and some good hikes up and down the mountains. You can also go in the castles and climb on the gates. History, nature, architecture, views, it’s as good as it gets.